Modern base station antennas use diversity techniques to reduce the effects of multipath fading. The most common diversity techniques are space and polarization diversities. Space diversity requires that the receiving antennas be placed far from each other so that independent samples of the incoming field are obtained. Hand-held phones transmit at both horizontal and vertical linear polarizations due to tilting of their antennas. The horizontal and vertical polarizations are uncorrelated. Polarization diversity is most commonly achieved by using dual-polarized (typically ±45° slanted polarizations) antennae at the base station. Nowadays, antennas using polarization diversity are preferred due to ease of installation and lower costs over space diversity antennas.
The base station antenna array consists of many antenna elements. The antenna elements should be easily manufacturable and should require minimum tuning during production. Further, the cost of used substrate materials of the antenna elements should be minimized. The RF (Radio Frequency) performance requirements are also very tight: a return loss better than 14 dB (Voltage Standing Wave Ratio,VSWR, <1.5) and a polarization isolation better than 30 dB are required.
Dual-polarized or dual-polarized aperture coupled, patch antennas are often used. Dipole antennas are broadband and many different dual-polarized dipole configurations have been developed. However, most dual-polarized dipole antenna configurations are mechanically complex due to the required feeding baluns for dipole branches. This results in high assembly costs due to manual labor, although the sheet material costs are low. Further, the dipole radiation patterns are typically very broad and shaped reflector ground planes are required for generating the desired beam patterns and for reducing back lobes.
Recently planar aperture-coupled patch antennas have also been developed. In this technique, a specially shaped aperture is etched into a ground plane between a microstrip feed line and a radiating patch. The dual feed lines required for dual polarizations can be arranged in different ways. Offset aperture slots are most commonly used and they can be coupled to either the patch diagonal or basic patch modes. A cross-shaped aperture can also give a good port isolation but the feed lines must be either on different layers or use non-symmetric arrangements. Deviation from symmetry usually then means increased cross-polarization and reduced port isolation.
Most recent dual-polarized patch antenna types are the L-probe fed microstrip patch antennas of Chinese origin. In the L-probe feeding technique, an L-shaped wire is located under the microstrip patch and it is used to proximity-couple the signal to the patch. However, the L-probe coupled antennas are mechanically more complex than aperture-coupled antennas due to the required air gaps and supports. The problem with dual-polarized L-probe fed antennas is the low polarization isolation resulting from a direct coupling between the vertical parts of the feed probes. New solutions for overcoming the aforementioned problems are needed.